This invention relates generally to laboratory fume hoods and more particularly to a method and apparatus for controlling the ventilation of a fume hood.
Many types of potentially harmful materials are handled in laboratories in fume hoods. An exhaust fan pulls air through the fume hood to make certain that fumes do not leak into the laboratory where they could be inhaled by personnel working in the vicinity. The materials in the fume hood are accessible through an opening which is controlled by a movable sash. The sash usually slides up and down to control the exposure of the fume hood opening.
In order to assure safe conditions, the velocity of the air entering the fume hood (referred to as the face velocity) must be maintained high enough to keep fumes from leaking into the room, yet it should not be so high that it creates turbulent conditions which can lead to the escape of contaminants. Thus, the face velocity should be maintained nearly constant and the velocity level should not vary appreciably throughout the normal working range of the sash. Since raising of the sash increases the effective size of the fume hood opening, the volume of air pulled through the hood must be increased in order to maintain a constant face velocity as the sash is raised. For most materials that are handled in fume hoods, a face velocity of approximately 100 feet per minute is satisfactory.
As the sash is lowered to restrict the hood opening, the volume rate of air flow can be decreased. However, maintaining a constant face velocity as the sash is lowered results in a progressively lower volume rate of air flow through the fume hood. When the sash is nearly closed, the volume of air flow can be so low that dangerous conditions can be created. Consequently, the volume rate of air flow through the fume hood should be maintained at or above a minimum level regardless of the position of the sash.
Another situation that can be dangerous occurs when the sash is opened abruptly. In this event, it takes some time for the face velocity to decrease and it takes additional time for the control system to sense the decreased face velocity and take appropriate action such as increasing the blower motor speed or opening a damper in the exhaust duct. The delay can be long enough to allow harmful fumes to escape from the hood.
Because the air which is drawn through the fume hood is heated or cooled air from within the building, the building energy requirements are increased with increasing amounts of air drawn through the fume hood. Therefore, it is desirable to minimize the air requirements of the hood, particularly overnight and on weekends when there are no personnel working in the laboratory and cut back of the air flow is feasible. However, it is important from a safety standpoint to make certain that the air flow is sufficient whenever the laboratory is occupied.
In the event of a blower failure or other malfunction in the exhaust system, the lack of ventilation of the fume hood can endanger personnel in the laboratory and they should be immediately alerted to the danger. Also, if power fails in an installation having a closed or partially closed damper in the exhaust duct, the restriction in the exhaust system can cause contaminants to leak into to the laboratory in large quantities. If an accidental spill or other emergency arises, contaminants are not quickly evacuated from the laboratory if the exhaust duct happens to be restricted at the time.